High-efficient video coding/decoding technology is the key of realizing storing and transmitting multimedia data at high quality and low cost. The present popular international standards for images coding are based on this coding theory which adopts coding method combining motion compensation based on block matching, discrete cosine transform and quantization. Typically, the first joint technology committee of international standards organization/International Electro-technical Commission (ISO/IEC JTC1) proposes motion picture experts group (namely to MPEG)-1, MPEG-2, MPEG-4 and such international standards; and the International Telecom Union (ITU-T) proposes the H.26x series. These video coding standards are widely used in the industries.
All these standards for video coding adopt Hybrid Video Coding strategy, normally including four main modules such as predicting, transforming, quantizing, information entropy coding etc. wherein, the main function of predicting module is to predict the current image to be coded by using the coded and reconstructed image, namely inter prediction, or to predict the current image part to be coded by using the coded and reconstructed image part in images, namely intra prediction. 
The main function of the prediction based on motion compensation is to reduce redundancy of video series on time. Most coding efficiency for video comes from the predicting module. The procedure of the video coding is to code each frame image of video series. The conventional video coding system which codes each frame image is based on macro block as a basic unit. When encoding the current macro block, the motion vector is involved in connecting the current macro block with the reference image block. When encoding each frame image, there are different situations which can be divided into intra coding (I frame), Prediction coding (P frame), bi-directional prediction (B frame) coding etc. Generally, when coding, I frame, P frame and B frame coding are interlarded, for example based on IBBPBBP sequence.
The B frame technology needs to process forward and backward motion estimation at the same time, so that higher computation complexity is needed. And in order to discriminate the forward motion vector from the backward motion vector, the additional flag information is needed to be introduced into. While using the direct coding mode, the forward and backward motion vectors are deduced from the motion vector having backward reference picture, and the motion vectors information might not be encoded. Therefore the bits for encoding the motion vectors information can be decreased and the coding efficiency can be effectively improved.
The deduction of the motion vectors under the direct mode in the prior art is realized as follows:MVF=td/tp×MV MVB=(tb−tp)/tp×MV 
MVF and MVB respectively denote the forward motion vector and the backward motion vector corresponding to the current block. Here, tb denotes the distance in time domain between the current picture and the forward reference picture. td denotes the distance in time domain  between the forward reference picture and the backward reference picture. MV denotes the motion vector of the corresponding part of the backward reference picture relative to the forward reference frame.
It can be figured out from the above formula that the values of MVF and MVB may not be integers. If the motion vector has decimal fraction, the matching block pointed by the motion vector can not be found in the reference picture because the motion vector is based on the pixel as a unit. Therefore, the calculation values of the MVF and MVB should be processed to be decided as integral motion vectors.
For instance, in the H.264/AVC, direct mode motion vectors MVF and MVB are calculated by the following formulas:Z=(tb×256)/tp W=Z−256MVF=(Z×MV+128)>>8MVB=(W×MV+128)>>8
Where “>>8” denotes right shift 8 bits.
However, the prediction block of the current block still can not be precisely obtained by this rounding method so that bias appear to the image block pointed by the obtained motion vector, which seriously influences the accuracy of deduction of the motion vectors in direct mode, while the direct mode often occupies 70%-80% in the modes used by the B frame so that the compression efficiency of B frame is greatly lowered. Therefore, how to determine motion vectors in direct mode becomes a problem in the video compression field.